Hi Ian

Sounds like you are having fun!

- I would suggest that using a GDO for meaningful antenna work at UHF
would incur quite a few inaccuracies. The big problem is how to couple
it to the element itself without detuning it by its presence. You will
hear some howls of protest but I personally wouldn't bother to try
making it a resonant length. It's really not that important for
radiation efficiency. The gamma match introduces a huge reactance that
also has to be allowed for.

- Using Ocarc's loss calculator 300r ribbon runs about 2.5dB for
100ft/30m. 450r open window ribbon runs about 0.7dB for the same
distance! (http://www.ocarc.ca/coax.htm) RG213 (approx 1/2") coax is
around 5dB. Keep in mind that ribbon bends and proximity to objects will
cause problems that coax wont.

- Converting coax to a balanced feed at the antenna can be very easily
done with a 4:1 coax balun. This is basically a extra loop of 1/2 wave
length coax built like http://www.n-lemma.com/calcs/dipole/balun.htm You
will still need to match at the antenna in some way. Maybe replace the
gamma with a hairpin, use a delta match or maybe increase the number of
driven elements folded dipole style to get the Z up to 200 ohms. A yagi
feedpoint Z is going to be maybe 10-20r. One extra driven element
(simple folded dipole) will multiply that by 4 and a third element
another 4 times. That will be close enough to 200 ohms to work fine. You
can also play with the extra element diameters to vary the final feed Z.

- I personally wouldnt use a UHF TV balun without testing first. The
problem is quality of manufacture!

Hope you find this useful. Only some of my ideas!

Cheers Bob W5/VK2YQA


wrote:

To optimise the matching to the driven element I used a gamma
match, the reality is I don't know if the driven element is at resonant
frequency let alone correctly adjusting the gamma match.

I was just reading another posting on rec.radio.amateur.antenna,
where the author suggested using 300 ohm TV ribbon as it is less lossy
than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will
remove the need for a gamma match at the antenna end, but at the
transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will
be required. Would a normal TV set balun suffice for 0.5 watt of
transmit power?

To resonate the driven element I have contemplated using a UHF
Gate Dip Oscillator (GDO), has anyone succeed in doing this with
minimal test equipment such as a GDO? If the folded driven element
resonates at 476 MHz I will accept that it's impedence is 300 ohms.